For a settling tank of given volume, the lower the sedimentation speed of the particles to be extracted by the tank, the smaller the throughput of water which it is capable of filtering, sedimentation speed being lower for finer particles or for a smaller difference in density relative to the base liquid constituted by the water carrying the particles. In order to increase the throughput of particle-charged water which can be filtered per unit volume of settling tank, it is known that the collecting surfaces can be constituted by a large number of inclined parallel plates forming between them a large number of water circulation streams, either in counter-flow i.e. the water moves upwards while the gravitational movement of the particles relative to the water is downwards, or in parallel flow, i.e. all movement is downwards. Other things being equal a parallel flow stream, i.e. one in which flow takes place in parallel, allows finer particles to be deposited more rapidly than does a counter-flow stream.